Electrochemical fuel cells convert a fuel and an oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction of hydrogen and oxygen produces electric current and water as the reaction product.
In applications employing electrochemical fuel cells to supply electric current to a variable load, such as, for example, vehicular applications, it is desirable that the power generation system have a rapid response time. The power supplied to the load by the system should respond quickly to changes in the power demanded by the load, so that additional power is provided immediately upon an increase in the load demand and conversely power output is immediately reduced upon a decrease in the load demand.
Recently, efforts have been devoted to developing hydrogen/oxygen fuel cell based power generation systems for vehicular, utility, industrial and residential applications using hydrogen derived from hydrocarbon conversion or reformation processes as the fuel. In such applications, the use of substantially pure hydrogen is disadvantageous because of the expense of producing and storing pure hydrogen gas and the general availability of hydrocarbon fuel sources such as natural gas and methanol. In addition, the use of liquid fuels is preferable to pure, bottled hydrogen in mobile and vehicular applications because liquid fuels are generally easier and safer to transport and store than gaseous fuels.
Conversion of hydrocarbons to hydrogen is generally accomplished through the steam reformation of a hydrocarbon such as methanol in a reactor sometimes referred to as a reformer. The steam reformation of methanol is represented by the following chemical equation: EQU CH.sub.3 OH+H.sub.2 O+heat.revreaction.3 H.sub.2 +CO.sub.2 ( 1)
Due to competing reactions, the initial gaseous mixture produced by steam reformation of methanol typically contains from about 65% to about 75% hydrogen, along with about 10% to about 25% carbon dioxide, and about 0.5% to about 20% by volume of carbon monoxide, on a dry basis (water vapor can also be present in the gas stream). The relatively small amount of carbon monoxide in the initial reformate gas mixture produced by the steam reformer can be further reduced by a selective oxidizing reactor to a level sufficiently low that the resulting hydrogen-containing reformate gas mixture can then be employed as the fuel source in hydrogen/oxygen fuel cells. In a selective oxidizing reactor, oxygen or an oxygen-containing gas mixture is introduced at locations along an isothermal reaction chamber containing catalyst to selectively oxidize the carbon monoxide to carbon dioxide and to suppress the reverse water-shift reaction, which produces carbon monoxide and water from carbon dioxide and hydrogen.
Fuel cell based power generation systems operating on reformed fuel can be viewed as a series of interconnected mechanical components and reactors. One such component is the vaporizer, which converts a raw liquid fuel source, such as methanol and water, to a vapor for subsequent conversion in the steam reformer to hydrogen and carbon dioxide, as set forth in equation (1) above. Since the response time of the overall power generation system is dependent upon the response time of each individual component, the vaporizer component should exhibit a minimal response time itself, and provide an increased vapor flow immediately upon an increase in the load demand.
Two approaches have been considered in the development of a vaporizer with a rapid response time: (1) a pressurized boiler, and (2) a once-through vaporizer. The pressurized boiler approach is considered to be less preferred because of the complexity of control and the inherent danger of a boiler containing a combustible hydrocarbon mixture.
The vaporizer disclosed herein employs a thermal fluid to distribute heat to the various heat transfer structures that first vaporize the liquid reactant fuel mixture and then superheat the vaporized mixture. The present vaporizer is designed to become part of an integrated fuel processing system, and has a geometry that is compatible with other fuel processing system components, such as the steam reformer and the selective oxidizing reactor discussed above.
The present vaporizer comprises three major components: a containment shell, a nozzle, and a fin block. An evaporator cap and a superheater cap segregate the respective evaporation and superheating chambers on either side of the fin block within the containment shell. In the embodiment disclosed herein, the vaporizer components are consolidated between two end plates or flanges. In an integrated fuel processing system, the vaporizer described herein will rely upon the end structure of the adjacent system components for consolidation and pressure containment.
One of the important principles underlying the vaporizer design disclosed herein is the avoidance of the pooling of liquids within the heated environment. Any such pooling represents a liquid inventory, and the size of the liquid inventory is a function of both the flow rate of the reactant mixture through the vaporizer and the heat transfer rate from the thermal fluid to the heat transfer structures of the vaporizer. Thus, the size of the liquid inventory will vary under different load conditions, and there will be a time lag between a change of the inlet liquid reactant flow rate and a corresponding change in the output vapor flow rate. This time lag, together with similar delays in other reactor components, contributes to a decrease in the overall response time of the fuel processing system, of which the vaporizer is a part. In order to be load-following, therefore, the time lag resulting from the presence of a liquid inventory should be minimized.
To avoid pooling, liquid entering the vaporizer should substantially instantaneously contact a heated surface with sufficient heat capacity to vaporize the liquid. The optimal vaporizer will therefore expose entering liquid to a maximum area of heated surface, at the highest temperature, and with a maximum heat capacity.
Accordingly, it is an object of the present invention to provide a load-following vaporizer that exhibits a minimal time lag between a change of the inlet liquid reactant flow rate and a corresponding change in the output vapor flow rate.
It is also an object of the invention to provide a load-following vaporizer that minimizes the liquid inventory within the heated environment.
It is a further object of the invention to provide to provide a load-following vaporizer that is compact, lightweight and compatible with other components of an integrated fuel processing system.